The Double Negative team used in-house fluid-simulation tool Squirt to create the Elemental’s coat of vines, leaves, as well as for adding the water vapour, water dips and smoke that helped it to integrate with the live-action plates.

Squirt was used in the Elemental death scene too, to simulate the blood or goo that flows from the fatal gunshot wounds Hellboy delivers. As the creature lies dying, anything this substance touches is quickly covered in moss, followed by flowers and plants, while the Elemental’s head becomes a huge orchid orchidlike flower, which releases clouds of pollen into the air as it dies.

“Technically that sequence was a nightmare because of the density of layers and effects that go into it,” says Chapman. “Once the Elemental is on the move there are dozens of layers going into those comps.

"For example, you might have the plate background of the street, additional crowd elements, digital set top-up, the Elemental itself comprising dozens of multi-pass render outputs, steam, drips, drool, helicopters, helicopter beams, digital vehicles flying, debris from the vehicles and the buildings, smoke, fire... It was crazy the amount of work that went into some of those shots. “But the fun thing about the Elemental sequence was that the guys involved were just so willing to go the extra mile to help make it look amazing,” says Chapman.

“They were always adding to the shots even though it meant more work for themselves. And the 2D team, led by Sean Stranks, did an incredible job of coming up with quick 2D solutions, rather than needing additional elements from 3D.”

Another huge VFX sequence was the fi lm’s finale, where the robotic Golden Army is awoken by Prince Nuada. Double Negative pre-vizualised and designed the robot soldiers, which are driven by animated internal machinery that glows with internal firelight, heat distortion and CG steam.

Initially the team had worked from a physical prototype created by Spectral Motion, but part-way through the build del Toro changed his mind on the design, which meant the characters had to be reworked entirely.

“I’m glad we did it, in the end they’re a much nicer design – kind of gorilla-like, as opposed to their initial more basic robotic look – but it had a massive impact for the team,” recalls Chapman.

According to Chapman, the Golden Army sequence was the only aspect of the film that the director hadn’t quite visualized, even up to the point of filming.

“We were designing that sequence as we went, during filming and right into post,” he says. “We got pretty heavily into postvisualization work here, which was a little frightening as some of the shots were post-vis, using the plates that were shot, but the urge was always to push into a full-digital environment.

“So in the end, there are a bunch of all-digital shots, as well as blends and handovers between different plates, and from plates to digital and vice versa.”

He continues: “They also suffered a lot of changes to the edit quite late in the process, which to the team’s credit they were able to absorb.” The final creative challenge for that sequence was the compositing of the robots.

“The problem is that you have these massive CG things in a static environment, you’re seeing them shot after shot under the same lighting, with the same backgrounds, so you can’t get away with much in terms of how well they sit into the plates,” Chapman explains.

Fighting fairies

The sequences featuring the tooth fairies may be the smallest of the film’s key sequences, but in many ways they were the hardest to complete, says Chapman.

“We definitely underestimated the amount of work involved,” he says. Reminiscent of the creatures from del Toro’s Pan Labyrinth, these creations have little in common with their dainty children’s story namesakes.

Instead they are fierce, vicious creatures with an insatiable appetite for calcium, and will happily eat their way though human flesh to get to bone and tooth.

Double Negative had to create thousands of fairies per shot, as they swarm out of holes in the wall and ceiling, enveloping and attacking the live actors. Several shots called for close-ups of individual fairies, such as the shot when Hellboy catches and squeezes a fairy until it explodes.

“It’s really quite challenging to have thousands of creatures in a shot and make sure that they also hold up well when there’s also a single one right in front of the lens,” explains Chapman.

“Doing one or the other is much easier, as it’s either a volume problem or a quality problem, and you use different techniques for each case – but when you have both volume and quality together, it becomes very difficult.”

The team used its proprietary particle system dnSwarm to create the multitude of Tooth Fairies. “It’s really just a front-end and some additional bits on top of Maya’s particle system,” says Chapman.

“It’s very easy to instance geometry onto particles in Maya, but there’s a bunch of additional expressions and setting up fields and things that you need to do on top of that to get them moving realistically and to be easily directed. DnSwarm packages up all that stuff to make it easy to block out the swarm shots.

“We had some experience with this kind of work from doing the locusts in The Reaping, but we knew it would have to be taken to a much higher level for Hellboy, so it was good to get it all wrapped up in a custom [system],” he says.

Blocks in the pipeline

Another big challenge of the Tooth Fairies sequence was the sheer volume of keyframed animation being passed between the animators and lighting artists.

The latter were using complex lighting on the sequence including sub-surface scattering which, when combined with the swarm animations and layered effects, saw the pipeline clog up.

“Thank goodness we had Jason Fairley working around the clock as a Pipeline TD on that sequence, so he was doing a lot of manual labour in massaging things back into line whenever it got a bit out of control,” says Chapman.

In addition to the big sequences, Double Negative worked on hundreds of smaller visual effects, from the creation of the Troll Market under Brooklyn Bridge with its menagerie of CG creatures required to fully interact with their surrounding to pyrotechnic Liz’s fire effects and the smoke trails and gases of new character Johann Kraus. Some of the smaller shots were just as rewarding as the major sequences, says Chapman.

“Sometimes, I would have to check the original plate to see what we had done – which of the creatures was practical or CG for example. For our own work to be invisible to us is pretty encouraging!

“It was a great bunch of artists we had working on this job, it was a joy to see some of the work as it emerged,” he says.

Despite the cute name, the film’s Tooth Fairies are deadly creatures that swarm over people and animals, stripping their flesh to reach the bones. Double Negative used its proprietary swarm technology to simulate thousands of these vicious fairies as they attack the film’s heroes.

An old flame

The fire that emanates from Liz – Hellboy’s pyrotechnic sweetheart, played by Selma Blair – was created in 3D.

The colourful Troll Market features a mix of CG creatures, digi-doubles and actors in prosthetic costumes.

Smashing robots

The Golden Army consists of 12-foot high mechanical robots each with a glowing core of light and heat, brought to life fully in CG by Double Negative. As the robots become damaged in the battle with Hellboy, the floor becomes littered with the cogs and machinery from inside them. One key moment in the sequence is a 3D fly-though of the cog landscape where the camera flies underneath the set to reveal a 3D subterranean world full of huge, red-hot cogs, foundry embers and steam.

The Double Negative team created the CG stone giant and incorporated him into the plates using a multitude of effects elements, as well as CG simulations of falling and shattering rocks, earth and sod. The live actors pass through the doorway in the stone giant’s belly and into the Bethmoora, the abandoned city.

One of the film’s new characters is Johann Kraus – an officious German who only exists as a vapour trail except when he’s bottled up in an archaic deep-sea diving suit. The main requirement when showing Johann as smoke when outside his suit was to make the smoke appear sentient as it snakes along.